that the velocity of fume in dust chambers shall not exceed 5 feet per second.) Draft in distributing flue of bag house, 1.11 in. of water. Pressure in bag house pits, 0.98 in. of water. Zinc fumes are filtered through cloth but it requires a closely woven one. Zinc-oxide fumes after proper cooling have little or no chemical effect on the filter cloth and the latter will last a long time. Woolen bags are preferred for this work.
FIG. 34.—Perfecto mechanically-operated bag filter.
Mechanically Operated Filter Bags.—Figure 34 shows a generalized view of the "Perfecto" filter. An accompanying table gives dimensions and capacities, these being reckoned for a single cylinder which forms the unit. There are usually 12 bags to the cylinder. There is a timing gear which automatically permits the flow of compressed air to each cylinder for a period of about 4 sec. When the compressed air flows towards the cylinder the piston controlling the reversing gate acts immediately. When the gate is shut the dust-laden air is cut off, the cylinder is open to the outside air and the dust-laden